1. Field of the Invention
This invention relates to an apparatus for ion implantation, and particularly relates to a structure of a wafer disc for holding wafers provided in the apparatus for executing ion implantation on wafers using a batch type production system for producing semiconductor devices.
2. Background Art
In a batch type ion implantation treatment apparatus, ion implantation is carried out for a plurality of wafers supported by a wafer disc with a large diameter. In general, the wafer disc comprises a plurality of wafer holders arranged around its periphery, and each wafer is supported and fixed in each wafer holder by a fixing means such as a clamp. In a practical ion implantation operation, the wafer disc is driven to rotate and to move in reciprocating motions about an ion beam incident perpendicularly on wafer surfaces such that a plurality of wafers on the wafer disc are subjected to homogeneous ion implantation.
However, the following problems are encountered in conventional batch-type ion implantation apparatuses.
This batch-type ion implantation apparatus comprises a fixed number of wafer holders, that is, the number of wafers which can be treated as one batch is fixed. However, in practical ion implantation operations, the number of wafers to be treated is not always the same as the number of wafer holders, and sometimes the number of wafers is less than the number of fixed wafer holders. In this case, since the ion beam scans the whole surface area of the wafer disc, it is necessary to cover vacant wafer holders by, for example, dummy wafers. The use of dummy wafers raises the cost per wafer, and causes inflation of the manufacturing cost of the wafers.
In addition, even though the number of wafers is less than the number of wafer holders, ion irradiates the whole peripheral surface area of the wafer disc including wafer holders, so that a problem arises that the ratio of the area of the product wafers to the total area of ion irradiation is low, which results in causing a low through-put.
Furthermore, even if it is desired to change the ion dose for wafers in one batch, the only way to do this in conventional ion implantation apparatuses is to provide wafers with different ion doses in different batches.
It is therefore an object of the present invention to solve the above described problems and to provide an apparatus for ion implantation which is capable of executing ion implantation without using dummy wafers even when the number of untreated wafers is less than the number of one full batch such that the manufacturing cost can be reduced and the through-put is improved, and also to provide an ion implantation apparatus which is capable of producing wafers with different ion doses in the same batch.